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The catalytic activation of primary alcohols on niobium oxide surfaces unraveled: the gas phase reactions of NbxOy− clusters with methanol and ethanol
Chemical Physics 262 (2000) 179±187
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The catalytic activation of primary alcohols on niobium oxide surfaces unraveled: the gas phase reactions of Nbx Oÿ y clusters with methanol and ethanol Phillip Jackson *, Keith J. Fisher, Gary D. Willett 1 The School of Chemistry, The University of New South Wales, Sydney, NSW 2052, Australia Received 12 September 2000
Abstract The reactions of oligomeric niobium oxide anions (up to Nb6 Oÿ 15 ), generated by laser ablation and studied using a Fourier transform ICR mass spectrometer, have been used to deduce the roles of (i) Nb(III,IV,V) centers, (ii) Nb/O double bonds and (iii) proximal Nb centers, in the catalytic activation of methanol and ethanol. The most important recurring mechanism involves initial alcohol condensation at a cluster metal-oxygen double bond to yield Nb(OH)(OCH3 ). There is no change in the oxidation state of the cluster during this step. The so-formed niobiumhydroxyl bond is the new reactive site in the cluster, and undergoes ligand switching in a follow-up collision to yield a bis-methoxy cluster and neutral water. Dehydrogenation is only observed to occur with clusters possessing two Nb/O double bonds at a single metal center, and involves reduction of the participating Nb(V) center to Nb(III). An ion ejection/selection step was used to monitor the activity of a number of the ionic reaction products towards the alcohols, and in most instances spontaneous or kinetically-activated decomposition resulted in regeneration of the parent cluster from the substituted species. Ó 2000 Elsevier Science B.V. All rights reserved. Keywords: Metal oxide anion clusters; Catalysis; Ion±molecule; Fourier transform ICR mass spectrometry
1. Introduction As the techniques of nanoscale engineering are improved, the reality of tailoring particle sizes for optimal performance in catalytic processes is nearer to fruition. The cost bene®ts that this might oer to large-scale operations are potentially enormous. Given that the importance of such devel*
Corresponding author. Present address: Research School of Chemistry, Australian National University, Canberra, ACT 0200, Australia. E-mail address: [email protected] (G.D. Willett). 1 Also corresponding author. Fax: +61-2-9385-6141.
opments is widely recognized, the study of metallic clusters in the gas phase by physical scientists has exploded over the last 15 years [1±3]. Somewhat surprisingly though, most research activity of catalytic relevance has concentrated on bare or ligated metal cations [4±8]. Recently, with the advent of commercial cluster sources [9], researchers are now probing much larger positively charged metallic clusters via ion±molecule chemistry [10± 12], but still there are only a handful of articles dealing with the ion±molecule chemistry of metalcontaining cluster anions [13±20]. Notably, the recent work of Shi and Ervin [21] has demonstrated that anionic platinum clusters will transfer
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a chemisorbed oxygen atom to an incoming CO molecule. The dearth of metal anion studies is undoubtedly due to the diculty of synthesizing ``low electron anity'' species in the gas phase. An added complication with gas phase metal studies is ®nding a suitably volatile precursor. This problem is exempli®ed for the refractory d1 ±d6 elements, for which researchers often resort to high temperatures and chemically extreme conditions to generate a sucient particle ¯ux for spectroscopic analysis. These are exactly the conditions under which anions, particularly those that might contain metals, are rarely observed. In the laser ablation Fourier transform ion cyclotron resonance [22] (LA-FT/ICR) mass spectrometer employed in this study, violent/rapid non-equilibrium heating, by means of a focused laser beam, is used to generate a ¯ux of Nbx Oy material from a pressed Nb2 O5 disk. The crucial dierence between this approach and other variants is that the solid target is ablated in the ICR cell of a FT/ICR mass spectrometer [23], so that a fraction of the photoejected electrons, which in a conventional cluster source are deposited on the walls of the source, are instead trapped by the combination of electric and magnetic ®elds in the ICR cell. This renders the charges available for capture by any gaseous species with a positive electron anity. We have demonstrated that such in cell laser ablation is particularly useful for generating anionic transition metal chalcogenide clusters [16,17], and has recently been applied to the study of anionic homonuclear main-group metal clusters [19,20]. 2. Experimental In the LA-FT/ICR mass spectrometry experiments, powdered niobium oxide (Nb2 O5 ) from Aldrich was compressed to a thickness of 2 mm in a detachable cylindrical (r 5 mm, h 10 mm) stainless steel FT/ICR probe tip. Experiments were performed on a Spectrospin CMS-47 mass spectrometer equipped with a 4.7 T superconducting magnet and a 128 K, 24 bit Aspect 3000 computer [23,24]. Typical, for the LA-FT/ICR MS experi-
ments, the 1064 nm Q-switch fundamental of a spectra-physics DCR-11 Nd-YAG laser was focused on to an area of 0.1 mm2 on the surface of the sample. The laser power was measured with a Scientech ED-500 power meter and Schott glass neutral density ®lters were used to vary the power densities at the sample. Typically it was greater than 15,000 MW cmÿ2 . Pulse programs similar to those shown previously were used for the collection of the mass spectra in the dierent LA-FT/ICR mass spectrometry experiments and are not discussed any further here [25,26]. 3. Results and discussion By LA-FT/ICR-MS it was possible to generate ÿ ÿ the anion clusters NbOÿ 3 , Nb2 O5 , Nb2 O6 H , ÿ ÿ ÿ ÿ Nb3 O8 , Nb4 O10 , Nb5 O13 , Nb6 O15 , etc. with genÿ eral stoichiometries Nb2 O5 n and Nb2 O5 n ± ÿ NbO2 , n 1±4, (Fig. 1). A very similar series of cationic niobium oxide clusters have been generated using ¯owing afterglow, and their reactions with a number of neutral reagents studied [27]. In this present study we focus on the chemistries of ÿ ÿ ÿ ÿ NbOÿ 3 , Nb2 O5 , Nb2 O6 H , Nb3 O8 and Nb4 O10 with the alcohols methanol and ethanol. Model Nb(V)-oxide monomers and dimers dispersed on silica supports have previously been studied using extended X-ray absorption ®ne structure [28], and the role of Nb(IV) centers in alcohol activations proposed on the basis of electron paramagnetic resonance measurements [29]. In both instances, only circumstantial evidence concerning the roles of particular Nb-moieties was obtained, and no information regarding reaction mechanisms was available. Due to the intrinsic molecular nature of the MS experiment, mechanistic information and variations in the activity with cluster size can be discerned. Moreover, due to the deceptively simple nature of the cluster structures, representative anions with the features implicated in Nb±O surface activity can be independently examined to test the spectroscopic paradigms. Due to the high strength of the Nb±O bond [30] and high oxygen: metal ratio, with few exceptions, notably Nb2 Oÿ 5 , all Nb atomic centers
P. Jackson et al. / Chemical Physics 262 (2000) 179±187
181
Fig. 1. Laser ablation Fourier transform negative-ion mass spectrum obtained from the laser-irradiation of a solid Nb2 O5 pellet. The measured laser power density was >15 000 MW cmÿ2 . The ®rst number in the peak labels corresponds to the number of niobium atoms, while the second number corresponds to the number of oxygen atoms.
will exist in the 5 oxidation state and the Nb±O bonding will dominate the cluster structure. We have recently reported the activity of NbOÿ 3 towards a variety of neutral molecules, and it is important to recount the results for the reactions of this ion with methanol and ethanol [26]. NbOÿ 3 dehydrogenates methanol in an ecient, concerted reaction to yield a relatively non-reactive Nb(III) product ion NbO(OH)ÿ 2 . Dehydration was a very minor reaction, which is probably due to the spin restrictions of the ground state product 3 CH2 . The aforementioned situation is reversed for the larger alcohol because the C2 H4 is a thermochemically stable closed shell neutral. Dehydrogenation was observed to compete with the dehydration reaction, but overall the activation of ethanol is far less ecient. An ion ejection/selection step was used to isolate the Nb(V) product ion from this reaction, NbO4 Hÿ 2 , which was observed to undergo an extremely inecient ligand switching reaction with ethanol to form NbO4 C2 Hÿ 6 (less than three reactions per one hundred collisions). Given that slow
or inecient activity is an indicator of a poor or poisoned catalyst, NbO4 Hÿ 2 was re-isolated and kinetically activated using RF irradiation. This resulted in regeneration of the more catalytically active parent ion NbOÿ 3 . Prior to interrogating this ion using gas phase chemistry, we were equipped with an a priori knowledge of its likely structure from local density functional calculations. In addition, O-atom loss was dominant in CID experiÿ ments, so a peroxo-type structure NbO O2 was immediately discounted. It is already clear that the MS experiments, even for our model monomer system, oers immense insights into Nb±O/alcohol surface chemistry, so we now investigate the reactions of a model Nb(IV) system, Nb2 Oÿ 5 . Simple empirical electron counting, and niobiumÕs preference for the 5 oxidation state, dictates that this cluster possesses a radical Nb(IV) center, as the electrophilicity of oxygen is much greater than that of niobium. (EA Nb 0:89 0:03 eV [31] cf. EA O 1:461 eV [32]). Predictably, it is the
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radical site and not the charge site that is dominant in the ®rst reaction of this cluster with both methanol and ethanol. That is, hydroxyl abstraction and liberation of the respective alkyl fragment in ecient, radical propagation reactions are observed /ADO;EtOH 0:47, and /ADO;MeOH 0:81. 2 Moreover, this is the sole reaction observed for Nb2 Oÿ 5 , and from general bond strength considerations D Nb2 Oÿ 5 ±OH > D CH3 ±OH 92:3 0:7 kcal molÿ1 [38±40]. The primary product ion Nb2 O6 Hÿ then undergoes a ligand switching reaction with methanol to yield methoxydiniobium pentoxide anion /ADO;MeOH 0:46 and the neutral by-product water. Although ligand switching is also observed for ethanol, this pathway competes less eciently with the protontransfer reaction /ADO;EtOH 0:75 that yields water, neutral diniobium pentoxide and presumably the ethoxy anion. Due to the nature of the experiment, in which only charged species are detected, we are left to speculate about the nature of the neutrals that are generated, and bishydroxydiniobium tetroxide could indeed be the undetected product of this reaction. Utilizing an ion ejection/selection step, two minor reactions of the secondary product were observed that either occur spontaneously, or require slight kinetic activation, and both are representative of diniobium pentoxide catalysis. The ®rst of these reactions is a rearrangement/elimination that yields neutral formaldehyde and hydroxydiniobium tetroxide anion in a formal redox process. This reaction may or may not require collision-induced activation with an additional methanol molecule. The second pathway is either a C±C or C±O coupling reaction that regenerates the parent ion Nb2 O6 Hÿ and produces either ethanol (chain propagation)
2 The / values are calculated by dividing the measured second-order rate constant by the calculated collision frequency. They are a measure of the reaction probability per collision. We have adopted the Ôaverage dipole orientationÕ (ADO) theory of Bowers and coworkers [33±36] as a suitable model for the prediction of collision frequencies. The molecular parameters used in these calculations (molecular dipole moments and polarisabilities) were taken from CRC Handbook of Chemistry and Physics [37].
or dimethyl ether. Given that ethers have been observed in the product gases of Nb2 O5 activated alcohols, the latter pathway is probably operative. After ion ejection/selection, the monomethoxydiniobium pentoxide anion was also observed to undergo an exothermic condensation reaction with methanol, followed by a further ligand switching reaction to ®nally yield a tris-methoxydiniobium tetroxide anion. This quaternary reaction product is either inert towards methanol, or further reaction requires thermal activation. At this point it would probably be prudent to regenerate the parent ion as the eciency of the alkoxydiniobium pentoxide reactions is reduced to 0.31 and 0.38 for methanol and ethanol respectively. Due to the presence of small amounts of adsorbed moisture on the precursor pellet, it was also possible to study the alcohol reactions of lasergenerated Nb2 O6 Hÿ . Interestingly, in addition to ligand switching, methanol oxidation was also facile for the primary parent ion. Assuming that alcohol oxidation reactivity is indicative of proximal metal-oxygen double bonds, geometric isomerism is a likely cause of this eect (Fig. 2). The branching ratio for laser-generated Nb2 O6 Hÿ reacting with methanol is dehydration (60%) and oxidation (40%). The reactions and rates of the isolated product ions are suciently close to those generated from Nb2 Oÿ 5 to suggest they are geometrically and electronically equivalent. Thus no elaboration of these results is necessary, although it should be mentioned that the yields of the oxidation product Nb2 O6 Hÿ 3 were signi®cantly lower for the ion±molecule reaction product Nb2 O6 Hÿ reacting in a further collision with CH3 OH. This is not unreasonable considering the possibility of generating a mixture of geometric isomers by the laser ablation/resonant electron capture method is high. We now extend our gas phase study to a cluster containing three niobium atoms, Nb3 Oÿ 8 , for which no information is available from earlier spectroscopic studies. It will be interesting to compare these results with those obtained for the smaller clusters, particularly for the appearance of size eects and any unexpected reactions that might be induced by the presence of the additional metal atom.
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183
Fig. 2. Geometric isomers, reaction pathways and branching ratio for laser-generated Nb2 O6 Hÿ .
In total, four molecules of CH3 OH are activated by Nb3 Oÿ 8 , and a recurrent mechanism appears to be operative which involves (i) a condensation reaction at an Nb@O bond to yield Nb(OH)(OCH3 )ÿ and (ii) a follow-up ligand switching reaction in the ensuing collision to yield Nb(OCH3 )ÿ 2 and water as the neutral by-product. The reactions with ethanol essentially follow the same scheme, but with niobium ethoxy species being the products of the reactions. We note that no methanol or ethanol oxidation is observed with this cluster, and reactivity terminates with the ®nal (slow) ligand switching reactions. The eciencies for each reaction step with CH3 OH are as follows: first condensation : /ADO 0:61; kexp 5:3 10ÿ10 cm3 sÿ1 ; first ligand switch : /ADO 0:32; kexp 2:8 10ÿ10 cm3 sÿ1 ; second condensation : /ADO 0:34; kexp 3:0 10ÿ10 cm3 sÿ1 ; second ligand switch : /ADO 0:04; kexp 3:3 10ÿ11 cm3 sÿ1 :
The last ligand switching reaction is notably slower than the preceding reactions, which all appear to follow statistical behavior for the probability of encountering a reactive or activated cluster site. That is to say, the ®rst condensation can take place at two of the three metal centers (probability 66%), whereas the follow-up ligand switching reaction can only take place at the activated cluster site with a probability of one in three (33%). We could extend this argument to the second condensation reaction, so the ®rst condensation/switching paci®es one of the metal centers towards further alcohol activation, leaving only one site available for further reaction. What is surprising is the ineciency of the ®nal switching reaction, for which we can cite hydroxyl ligand occlusion as a possible explanation (Fig. 3). Another interesting aspect of the reaction of Nb3 Oÿ 8 with both alcohols was the almost simultaneous appearance of the condensation product Nb3 Oÿ 8 ROH and the primary hydration product Nb3 Oÿ 8 H2 O. Using an ion ejection/selection step,
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ÿ Fig. 3. Geometric isomers derived from the observed cluster reactions of Nb3 Oÿ 8 , and the chemical fate of isolated Nb3 O9 CH4 upon further reaction with methanol: an example of molecular catalysis.
we were able to isolate the primary condensation product and examine subsequent reactions of this ion with the respective alcohols. Interestingly, the hydration product does indeed result from further reaction of the condensation product with the alcohols. This is probably via C±O coupling to form
ROR, where R is either methyl or ethyl. In addition, the hydration product decomposes to yield the parent ion without activation. This is a remarkable result, although we cannot completely exclude some kinetic energy being imparted to the parent ion by the ion ejection/selection process.
P. Jackson et al. / Chemical Physics 262 (2000) 179±187
Nevertheless, the direct observation of catalysis on a small, mass-selected cluster has been realized. Two structural possibilities for the Nb3 Oÿ 8 cluster appear in Fig. 3, together with the branching ratio for the reaction of the isolated ®rst condensation product. Catalysis reactions involving cluster cations by FT/ICR mass spectrometry have been reported previously by Irion and coworkers in the gas phase study of the trimerization of ethylene by Fe 4 clusters [41,42] and further discussed in a review by Irion on size eects in metal cluster-ion chemistry [43]. Continuing with our analysis of the higher mass clusters, we proceed to examine Nb4 Oÿ 10 , which we ®nd to be totally inert towards both alcohols, and a number of other reactive molecules including H2 S and N2 O. The high stability of this cluster, at least towards the alcohols, can be attributed to a closed-shell structure probably containing no Nb@O bonds, and a large HOMO±LUMO gap. We rely on the previous spectroscopic analyses of Shira et al. [28] and Wada et al. [29] as well as the established chemistry of the constituent elements for deductions regarding the cluster structure. Although the arguments presented pertaining to the cluster structure of Nb4 Oÿ 10 are compelling, they are not proven without recourse to high level ab initio calculations. At this point it appears that a structural bottleneck for further alcohol activation has been reached, as the higher clusters are either etched to produce smaller Nbx Oÿ y daughter clusters, or simply undergo slow condensation reactions, or both. For example, Nb5 Oÿ 13 undergoes a slow condensation reaction with CH3 OH, and the parent ion can be retrieved from this product by collision-induced decomposition. The relative ADO eciencies of some selected parent and product ions with both alcohols are presented in Fig. 4. It is clear that the monomeric and dimeric Nb clusters are the most reactive, with the only selectivity discrepancies being the Nb2 O6 Hÿ reaction with ethanol (acid±base) and NbOÿ 3 , which does not add H2 O and eliminate CH2 because of either unfavorable thermochemistry or a spin barrier. Finally, to ensure that many of the product ions were not irreversibly poisoned, a number of these were ®rst isolated using an ion ejection/selection
185
Fig. 4. Relative ADO reaction eciencies for the selected cluster/cluster products reacting with methanol and ethanol: R CH3 for methanol, R C2 H5 for ethanol.
step, and then accelerated with resonant radio frequency radiation to induce collisional fragmentation, thus replicating thermal reduction. For instance, isolation and resonant irradiation of the secondary product ion of the reaction between Nb2 O6 Hÿ and C2 H5 OH, Nb2 O7 C4 Hÿ 11 (bis-ethoxyhydroxydiniobium tetroxide anion) for 40 ls yields Nb2 O6 Hÿ and Nb2 O6 C2 Hÿ 5 in the approximate ratio 0.4:0.6. Thus, ligand coupling is taking place with a minimum of eort, but it appears that this reaction does require some heating to proceed. Increasing the irradiation time to 60 ls (thereby increasing the center-of-mass collision energy) results in a greater degree of parent ion fragmentation as outlined below:
This scheme suggests that a much richer cluster chemistry is accessible at higher temperatures, and that dehydration, some oxidation reactions and
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ÿ Fig. 5. Kinetic activation of Nb3 O9 C4 Hÿ 10 , derived from Nb3 O8 reacting with ethanol, after a resonant radio-frequency irradiation time of (A) 40 ls and (B) 60 ls. The parent ion is the only decomposition product. The decrease in the signal to noise ratio in (B) is attributed to increased competition from electron detachment at higher center-of-mass collision energies.
ligand switching are all exothermic or thermoneutral for most of the clusters under the conditions of our experiment. We present the collisionally activated regeneration of Nb3 Oÿ 8 from in Fig. 5. the tertiary product ion Nb3 O9 C4 Hÿ 10 This ®gure clearly illustrates there are no intermediate decomposition products, so it is apparent that ligand coupling proceeds on the cluster before activation. If it takes place during activation, statistical arguments could be used to rationalize a very small activation energy for this reaction.
We are currently exploring other anionic metal oxide systems for analogous behavior. Acknowledgements We gratefully acknowledge ®nancial support from the Australian Research Council, the Australian Institute of Nuclear Science and Engineering, and the Nuclear Medicine Research Foundation, Concord Hospital, Sydney, Australia.
P. Jackson et al. / Chemical Physics 262 (2000) 179±187
References [1] M. Moskovits, in: E.R. Bernstein (Ed.), Atomic and Molecular Clusters, Wiley, New York, 1986. [2] D.C. Parent, S.L. Anderson, Chem. Rev. 92 (1992) 1541. [3] M.F. Jarrold, E.J. Bower, J. Chem. Phys. 87 (1987) 1610. [4] K. Eller, H. Schwarz, Chem. Rev. 91 (1991) 1121. [5] D. Schr oder, H. Schwarz, Angew. Chem. Int. Ed. Engl. 34 (1995) 1973. [6] B.S. Freiser, Acc. Chem. Res. 27 (1994) 353. [7] B.S. Freiser, J. Mass Spectrom. 31 (1996) 703. [8] C.J. Cassady, D.A. Weil, S.W. McElvany, J. Chem. Phys. 96 (1992) 691. [9] S. Maruyama, L.R. Anderson, R.E. Smalley, Rev. Sci. Instrum. 61 (1990) 3686. [10] J.L. Elkind, F.D. Weiss, J.M. Alford, R.T. Laaksonen, R.E. Smalley, J. Chem. Phys. 88 (1988) 5215. [11] C. Berg, T. Schindler, G. Niedner-Schatteburg, V.E. Bondybey, J. Chem. Phys. 102 (1995) 4870. [12] C. Berg, M. Beyer, T. Schindler, G. Niedner-Schatteburg, V.E. Bondybey, J. Chem. Phys. 104 (1996) 7940. [13] R. Squires, Chem. Rev. 87 (1987) 623. [14] J.R. Gord, R.J. Bemish, B.S. Freiser, Int. J. Mass Ion Proc. 102 (1990) 115. [15] S. Maleknia, J. Brodbelt, K. Pope, J. Am. Soc. Mass Spectrom. 2 (1991) 212. [16] J.H. El-Nakat, I.G. Dance, K.J. Fisher, D. Rice, G.D. Willett, J. Am. Chem. Soc. 113 (1991) 5141. [17] J.H. El-Nakat, K.J. Fisher, I.G. Dance, G.D. Willett, Inorg. Chem. 32 (1993) 1931. [18] J.K. Gibson, J. Phys. Chem. 98 (1994) 11321. [19] P. Jackson, I.G. Dance, K.J. Fisher, G.D. Willett, G.E. Gadd, Int. J. Mass Spectrom. Ion Process 157/158 (1996) 329. [20] P. Jackson, I.G. Dance, K.J. Fisher, G.D. Willett, G.E. Gadd, Int. J. Mass Spectrom. Ion Phys. 164 (1997) 45. [21] Y. Shi, K. Ervin, J. Chem. Phys. 108 (1998) 1757. [22] A.G. Marshall, C.L. Hendrickson, G.S. Jackson, Mass Spectrom. Rev. 17 (1998) 1.
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[23] T.H. Nguyen, P.S. Clezy, G.L. Paul, J. Tan, G.D. Willett, P.J. Derrick, Org. Mass Spectrom. 26 (1991) 215. [24] P.F. Greenwood, M.G. Strachan, G.D. Willett, M.A. Wilson, Org. Mass Spectrom. 25 (1990) 353. [25] F.G. Hopwood, K.J. Fisher, P. Greenhill, G.D. Willett, R. Zhang, J. Phys. Chem. B 101 (50) (1997) 10704. [26] P. Jackson, K.J. Fisher, G.D. Willett, Int. J. Mass Spectrom. 197 (2000) 95. [27] H.T. Deng, K.P. Kerns, J.A.W. Castleman, J. Phys. Chem. 100 (1996) 13386. [28] M. Shirai, N. Ichikuni, K. Asakura, Y. Iwasawa, Catal. Today 8 (1990) 57. [29] Y. Wada, A. Morikawa, Catal. Today 8 (1990) 13. [30] M.R. Sievers, Y.-M. Chen, P.B. Armentrout, J. Chem. Phys. 105 (1996) 6322. [31] H. Hotop, W.C. Lineberger, J. Phys. Chem. Ref. Data 14 (1985) 731. [32] D.M. Neumark, K.R. Lykke, T. Andersen, W.C. Lineberger, Phys. Rev. 32 A (1980) 1890. [33] M.T. Bowers, J.B. Laudenslanger, J. Chem. Phys. 56 (1972) 4711. [34] T. Su, M.T. Bowers, J. Chem. Phys. 58 (1973) 3027. [35] T. Su, M.T. Bowers, Int. J. Mass Spectrom. Ion Phys. 12 (1973) 347. [36] T. Su, M.T. Bowers, Int. J. Mass Spectrom. Ion Phys. 17 (1975) 211. [37] R.C. Weast (Ed.), CRC Handbook of Chemistry and Physics, CRC Press, Boca Raton, FL, 1985. [38] J.B. Pedley, J. Rylance, Computer-analysed Thermochemical Data: Organic and Organometallic Compounds, University of Sussex, Brighton, 1977. [39] M.W. Chase, C.A. Davies, J.J.R. Downey, D.J. Frurip, JANAF Thermochemical Tables, J. Phys. Chem. Ref. Data (Suppl. 1) 14 (1985) 1. [40] S.P. Henegan, P.A. Knoot, S.W. Benson, Int. J. Chem. Kinet. 13 (1981) 677. [41] P. Schnabel, M.P. Irion, K.G. Weil, J. Phys. Chem. 95 (1991) 9688. [42] P. Schnabel, M.P. Irion, K.G. Weil, Chem. Phys. Lett. 190 (1992) 255. [43] M.P. Irion, Int. J. Mass Spectrom. Ion Process 121 (1992) 1.
www.elsevier.nl/locate/chemphys
The catalytic activation of primary alcohols on niobium oxide surfaces unraveled: the gas phase reactions of Nbx Oÿ y clusters with methanol and ethanol Phillip Jackson *, Keith J. Fisher, Gary D. Willett 1 The School of Chemistry, The University of New South Wales, Sydney, NSW 2052, Australia Received 12 September 2000
Abstract The reactions of oligomeric niobium oxide anions (up to Nb6 Oÿ 15 ), generated by laser ablation and studied using a Fourier transform ICR mass spectrometer, have been used to deduce the roles of (i) Nb(III,IV,V) centers, (ii) Nb/O double bonds and (iii) proximal Nb centers, in the catalytic activation of methanol and ethanol. The most important recurring mechanism involves initial alcohol condensation at a cluster metal-oxygen double bond to yield Nb(OH)(OCH3 ). There is no change in the oxidation state of the cluster during this step. The so-formed niobiumhydroxyl bond is the new reactive site in the cluster, and undergoes ligand switching in a follow-up collision to yield a bis-methoxy cluster and neutral water. Dehydrogenation is only observed to occur with clusters possessing two Nb/O double bonds at a single metal center, and involves reduction of the participating Nb(V) center to Nb(III). An ion ejection/selection step was used to monitor the activity of a number of the ionic reaction products towards the alcohols, and in most instances spontaneous or kinetically-activated decomposition resulted in regeneration of the parent cluster from the substituted species. Ó 2000 Elsevier Science B.V. All rights reserved. Keywords: Metal oxide anion clusters; Catalysis; Ion±molecule; Fourier transform ICR mass spectrometry
1. Introduction As the techniques of nanoscale engineering are improved, the reality of tailoring particle sizes for optimal performance in catalytic processes is nearer to fruition. The cost bene®ts that this might oer to large-scale operations are potentially enormous. Given that the importance of such devel*
Corresponding author. Present address: Research School of Chemistry, Australian National University, Canberra, ACT 0200, Australia. E-mail address: [email protected] (G.D. Willett). 1 Also corresponding author. Fax: +61-2-9385-6141.
opments is widely recognized, the study of metallic clusters in the gas phase by physical scientists has exploded over the last 15 years [1±3]. Somewhat surprisingly though, most research activity of catalytic relevance has concentrated on bare or ligated metal cations [4±8]. Recently, with the advent of commercial cluster sources [9], researchers are now probing much larger positively charged metallic clusters via ion±molecule chemistry [10± 12], but still there are only a handful of articles dealing with the ion±molecule chemistry of metalcontaining cluster anions [13±20]. Notably, the recent work of Shi and Ervin [21] has demonstrated that anionic platinum clusters will transfer
0301-0104/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 0 1 0 4 ( 0 0 ) 0 0 3 0 4 - 9
180
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a chemisorbed oxygen atom to an incoming CO molecule. The dearth of metal anion studies is undoubtedly due to the diculty of synthesizing ``low electron anity'' species in the gas phase. An added complication with gas phase metal studies is ®nding a suitably volatile precursor. This problem is exempli®ed for the refractory d1 ±d6 elements, for which researchers often resort to high temperatures and chemically extreme conditions to generate a sucient particle ¯ux for spectroscopic analysis. These are exactly the conditions under which anions, particularly those that might contain metals, are rarely observed. In the laser ablation Fourier transform ion cyclotron resonance [22] (LA-FT/ICR) mass spectrometer employed in this study, violent/rapid non-equilibrium heating, by means of a focused laser beam, is used to generate a ¯ux of Nbx Oy material from a pressed Nb2 O5 disk. The crucial dierence between this approach and other variants is that the solid target is ablated in the ICR cell of a FT/ICR mass spectrometer [23], so that a fraction of the photoejected electrons, which in a conventional cluster source are deposited on the walls of the source, are instead trapped by the combination of electric and magnetic ®elds in the ICR cell. This renders the charges available for capture by any gaseous species with a positive electron anity. We have demonstrated that such in cell laser ablation is particularly useful for generating anionic transition metal chalcogenide clusters [16,17], and has recently been applied to the study of anionic homonuclear main-group metal clusters [19,20]. 2. Experimental In the LA-FT/ICR mass spectrometry experiments, powdered niobium oxide (Nb2 O5 ) from Aldrich was compressed to a thickness of 2 mm in a detachable cylindrical (r 5 mm, h 10 mm) stainless steel FT/ICR probe tip. Experiments were performed on a Spectrospin CMS-47 mass spectrometer equipped with a 4.7 T superconducting magnet and a 128 K, 24 bit Aspect 3000 computer [23,24]. Typical, for the LA-FT/ICR MS experi-
ments, the 1064 nm Q-switch fundamental of a spectra-physics DCR-11 Nd-YAG laser was focused on to an area of 0.1 mm2 on the surface of the sample. The laser power was measured with a Scientech ED-500 power meter and Schott glass neutral density ®lters were used to vary the power densities at the sample. Typically it was greater than 15,000 MW cmÿ2 . Pulse programs similar to those shown previously were used for the collection of the mass spectra in the dierent LA-FT/ICR mass spectrometry experiments and are not discussed any further here [25,26]. 3. Results and discussion By LA-FT/ICR-MS it was possible to generate ÿ ÿ the anion clusters NbOÿ 3 , Nb2 O5 , Nb2 O6 H , ÿ ÿ ÿ ÿ Nb3 O8 , Nb4 O10 , Nb5 O13 , Nb6 O15 , etc. with genÿ eral stoichiometries Nb2 O5 n and Nb2 O5 n ± ÿ NbO2 , n 1±4, (Fig. 1). A very similar series of cationic niobium oxide clusters have been generated using ¯owing afterglow, and their reactions with a number of neutral reagents studied [27]. In this present study we focus on the chemistries of ÿ ÿ ÿ ÿ NbOÿ 3 , Nb2 O5 , Nb2 O6 H , Nb3 O8 and Nb4 O10 with the alcohols methanol and ethanol. Model Nb(V)-oxide monomers and dimers dispersed on silica supports have previously been studied using extended X-ray absorption ®ne structure [28], and the role of Nb(IV) centers in alcohol activations proposed on the basis of electron paramagnetic resonance measurements [29]. In both instances, only circumstantial evidence concerning the roles of particular Nb-moieties was obtained, and no information regarding reaction mechanisms was available. Due to the intrinsic molecular nature of the MS experiment, mechanistic information and variations in the activity with cluster size can be discerned. Moreover, due to the deceptively simple nature of the cluster structures, representative anions with the features implicated in Nb±O surface activity can be independently examined to test the spectroscopic paradigms. Due to the high strength of the Nb±O bond [30] and high oxygen: metal ratio, with few exceptions, notably Nb2 Oÿ 5 , all Nb atomic centers
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181
Fig. 1. Laser ablation Fourier transform negative-ion mass spectrum obtained from the laser-irradiation of a solid Nb2 O5 pellet. The measured laser power density was >15 000 MW cmÿ2 . The ®rst number in the peak labels corresponds to the number of niobium atoms, while the second number corresponds to the number of oxygen atoms.
will exist in the 5 oxidation state and the Nb±O bonding will dominate the cluster structure. We have recently reported the activity of NbOÿ 3 towards a variety of neutral molecules, and it is important to recount the results for the reactions of this ion with methanol and ethanol [26]. NbOÿ 3 dehydrogenates methanol in an ecient, concerted reaction to yield a relatively non-reactive Nb(III) product ion NbO(OH)ÿ 2 . Dehydration was a very minor reaction, which is probably due to the spin restrictions of the ground state product 3 CH2 . The aforementioned situation is reversed for the larger alcohol because the C2 H4 is a thermochemically stable closed shell neutral. Dehydrogenation was observed to compete with the dehydration reaction, but overall the activation of ethanol is far less ecient. An ion ejection/selection step was used to isolate the Nb(V) product ion from this reaction, NbO4 Hÿ 2 , which was observed to undergo an extremely inecient ligand switching reaction with ethanol to form NbO4 C2 Hÿ 6 (less than three reactions per one hundred collisions). Given that slow
or inecient activity is an indicator of a poor or poisoned catalyst, NbO4 Hÿ 2 was re-isolated and kinetically activated using RF irradiation. This resulted in regeneration of the more catalytically active parent ion NbOÿ 3 . Prior to interrogating this ion using gas phase chemistry, we were equipped with an a priori knowledge of its likely structure from local density functional calculations. In addition, O-atom loss was dominant in CID experiÿ ments, so a peroxo-type structure NbO O2 was immediately discounted. It is already clear that the MS experiments, even for our model monomer system, oers immense insights into Nb±O/alcohol surface chemistry, so we now investigate the reactions of a model Nb(IV) system, Nb2 Oÿ 5 . Simple empirical electron counting, and niobiumÕs preference for the 5 oxidation state, dictates that this cluster possesses a radical Nb(IV) center, as the electrophilicity of oxygen is much greater than that of niobium. (EA Nb 0:89 0:03 eV [31] cf. EA O 1:461 eV [32]). Predictably, it is the
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radical site and not the charge site that is dominant in the ®rst reaction of this cluster with both methanol and ethanol. That is, hydroxyl abstraction and liberation of the respective alkyl fragment in ecient, radical propagation reactions are observed /ADO;EtOH 0:47, and /ADO;MeOH 0:81. 2 Moreover, this is the sole reaction observed for Nb2 Oÿ 5 , and from general bond strength considerations D Nb2 Oÿ 5 ±OH > D CH3 ±OH 92:3 0:7 kcal molÿ1 [38±40]. The primary product ion Nb2 O6 Hÿ then undergoes a ligand switching reaction with methanol to yield methoxydiniobium pentoxide anion /ADO;MeOH 0:46 and the neutral by-product water. Although ligand switching is also observed for ethanol, this pathway competes less eciently with the protontransfer reaction /ADO;EtOH 0:75 that yields water, neutral diniobium pentoxide and presumably the ethoxy anion. Due to the nature of the experiment, in which only charged species are detected, we are left to speculate about the nature of the neutrals that are generated, and bishydroxydiniobium tetroxide could indeed be the undetected product of this reaction. Utilizing an ion ejection/selection step, two minor reactions of the secondary product were observed that either occur spontaneously, or require slight kinetic activation, and both are representative of diniobium pentoxide catalysis. The ®rst of these reactions is a rearrangement/elimination that yields neutral formaldehyde and hydroxydiniobium tetroxide anion in a formal redox process. This reaction may or may not require collision-induced activation with an additional methanol molecule. The second pathway is either a C±C or C±O coupling reaction that regenerates the parent ion Nb2 O6 Hÿ and produces either ethanol (chain propagation)
2 The / values are calculated by dividing the measured second-order rate constant by the calculated collision frequency. They are a measure of the reaction probability per collision. We have adopted the Ôaverage dipole orientationÕ (ADO) theory of Bowers and coworkers [33±36] as a suitable model for the prediction of collision frequencies. The molecular parameters used in these calculations (molecular dipole moments and polarisabilities) were taken from CRC Handbook of Chemistry and Physics [37].
or dimethyl ether. Given that ethers have been observed in the product gases of Nb2 O5 activated alcohols, the latter pathway is probably operative. After ion ejection/selection, the monomethoxydiniobium pentoxide anion was also observed to undergo an exothermic condensation reaction with methanol, followed by a further ligand switching reaction to ®nally yield a tris-methoxydiniobium tetroxide anion. This quaternary reaction product is either inert towards methanol, or further reaction requires thermal activation. At this point it would probably be prudent to regenerate the parent ion as the eciency of the alkoxydiniobium pentoxide reactions is reduced to 0.31 and 0.38 for methanol and ethanol respectively. Due to the presence of small amounts of adsorbed moisture on the precursor pellet, it was also possible to study the alcohol reactions of lasergenerated Nb2 O6 Hÿ . Interestingly, in addition to ligand switching, methanol oxidation was also facile for the primary parent ion. Assuming that alcohol oxidation reactivity is indicative of proximal metal-oxygen double bonds, geometric isomerism is a likely cause of this eect (Fig. 2). The branching ratio for laser-generated Nb2 O6 Hÿ reacting with methanol is dehydration (60%) and oxidation (40%). The reactions and rates of the isolated product ions are suciently close to those generated from Nb2 Oÿ 5 to suggest they are geometrically and electronically equivalent. Thus no elaboration of these results is necessary, although it should be mentioned that the yields of the oxidation product Nb2 O6 Hÿ 3 were signi®cantly lower for the ion±molecule reaction product Nb2 O6 Hÿ reacting in a further collision with CH3 OH. This is not unreasonable considering the possibility of generating a mixture of geometric isomers by the laser ablation/resonant electron capture method is high. We now extend our gas phase study to a cluster containing three niobium atoms, Nb3 Oÿ 8 , for which no information is available from earlier spectroscopic studies. It will be interesting to compare these results with those obtained for the smaller clusters, particularly for the appearance of size eects and any unexpected reactions that might be induced by the presence of the additional metal atom.
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183
Fig. 2. Geometric isomers, reaction pathways and branching ratio for laser-generated Nb2 O6 Hÿ .
In total, four molecules of CH3 OH are activated by Nb3 Oÿ 8 , and a recurrent mechanism appears to be operative which involves (i) a condensation reaction at an Nb@O bond to yield Nb(OH)(OCH3 )ÿ and (ii) a follow-up ligand switching reaction in the ensuing collision to yield Nb(OCH3 )ÿ 2 and water as the neutral by-product. The reactions with ethanol essentially follow the same scheme, but with niobium ethoxy species being the products of the reactions. We note that no methanol or ethanol oxidation is observed with this cluster, and reactivity terminates with the ®nal (slow) ligand switching reactions. The eciencies for each reaction step with CH3 OH are as follows: first condensation : /ADO 0:61; kexp 5:3 10ÿ10 cm3 sÿ1 ; first ligand switch : /ADO 0:32; kexp 2:8 10ÿ10 cm3 sÿ1 ; second condensation : /ADO 0:34; kexp 3:0 10ÿ10 cm3 sÿ1 ; second ligand switch : /ADO 0:04; kexp 3:3 10ÿ11 cm3 sÿ1 :
The last ligand switching reaction is notably slower than the preceding reactions, which all appear to follow statistical behavior for the probability of encountering a reactive or activated cluster site. That is to say, the ®rst condensation can take place at two of the three metal centers (probability 66%), whereas the follow-up ligand switching reaction can only take place at the activated cluster site with a probability of one in three (33%). We could extend this argument to the second condensation reaction, so the ®rst condensation/switching paci®es one of the metal centers towards further alcohol activation, leaving only one site available for further reaction. What is surprising is the ineciency of the ®nal switching reaction, for which we can cite hydroxyl ligand occlusion as a possible explanation (Fig. 3). Another interesting aspect of the reaction of Nb3 Oÿ 8 with both alcohols was the almost simultaneous appearance of the condensation product Nb3 Oÿ 8 ROH and the primary hydration product Nb3 Oÿ 8 H2 O. Using an ion ejection/selection step,
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ÿ Fig. 3. Geometric isomers derived from the observed cluster reactions of Nb3 Oÿ 8 , and the chemical fate of isolated Nb3 O9 CH4 upon further reaction with methanol: an example of molecular catalysis.
we were able to isolate the primary condensation product and examine subsequent reactions of this ion with the respective alcohols. Interestingly, the hydration product does indeed result from further reaction of the condensation product with the alcohols. This is probably via C±O coupling to form
ROR, where R is either methyl or ethyl. In addition, the hydration product decomposes to yield the parent ion without activation. This is a remarkable result, although we cannot completely exclude some kinetic energy being imparted to the parent ion by the ion ejection/selection process.
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Nevertheless, the direct observation of catalysis on a small, mass-selected cluster has been realized. Two structural possibilities for the Nb3 Oÿ 8 cluster appear in Fig. 3, together with the branching ratio for the reaction of the isolated ®rst condensation product. Catalysis reactions involving cluster cations by FT/ICR mass spectrometry have been reported previously by Irion and coworkers in the gas phase study of the trimerization of ethylene by Fe 4 clusters [41,42] and further discussed in a review by Irion on size eects in metal cluster-ion chemistry [43]. Continuing with our analysis of the higher mass clusters, we proceed to examine Nb4 Oÿ 10 , which we ®nd to be totally inert towards both alcohols, and a number of other reactive molecules including H2 S and N2 O. The high stability of this cluster, at least towards the alcohols, can be attributed to a closed-shell structure probably containing no Nb@O bonds, and a large HOMO±LUMO gap. We rely on the previous spectroscopic analyses of Shira et al. [28] and Wada et al. [29] as well as the established chemistry of the constituent elements for deductions regarding the cluster structure. Although the arguments presented pertaining to the cluster structure of Nb4 Oÿ 10 are compelling, they are not proven without recourse to high level ab initio calculations. At this point it appears that a structural bottleneck for further alcohol activation has been reached, as the higher clusters are either etched to produce smaller Nbx Oÿ y daughter clusters, or simply undergo slow condensation reactions, or both. For example, Nb5 Oÿ 13 undergoes a slow condensation reaction with CH3 OH, and the parent ion can be retrieved from this product by collision-induced decomposition. The relative ADO eciencies of some selected parent and product ions with both alcohols are presented in Fig. 4. It is clear that the monomeric and dimeric Nb clusters are the most reactive, with the only selectivity discrepancies being the Nb2 O6 Hÿ reaction with ethanol (acid±base) and NbOÿ 3 , which does not add H2 O and eliminate CH2 because of either unfavorable thermochemistry or a spin barrier. Finally, to ensure that many of the product ions were not irreversibly poisoned, a number of these were ®rst isolated using an ion ejection/selection
185
Fig. 4. Relative ADO reaction eciencies for the selected cluster/cluster products reacting with methanol and ethanol: R CH3 for methanol, R C2 H5 for ethanol.
step, and then accelerated with resonant radio frequency radiation to induce collisional fragmentation, thus replicating thermal reduction. For instance, isolation and resonant irradiation of the secondary product ion of the reaction between Nb2 O6 Hÿ and C2 H5 OH, Nb2 O7 C4 Hÿ 11 (bis-ethoxyhydroxydiniobium tetroxide anion) for 40 ls yields Nb2 O6 Hÿ and Nb2 O6 C2 Hÿ 5 in the approximate ratio 0.4:0.6. Thus, ligand coupling is taking place with a minimum of eort, but it appears that this reaction does require some heating to proceed. Increasing the irradiation time to 60 ls (thereby increasing the center-of-mass collision energy) results in a greater degree of parent ion fragmentation as outlined below:
This scheme suggests that a much richer cluster chemistry is accessible at higher temperatures, and that dehydration, some oxidation reactions and
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ÿ Fig. 5. Kinetic activation of Nb3 O9 C4 Hÿ 10 , derived from Nb3 O8 reacting with ethanol, after a resonant radio-frequency irradiation time of (A) 40 ls and (B) 60 ls. The parent ion is the only decomposition product. The decrease in the signal to noise ratio in (B) is attributed to increased competition from electron detachment at higher center-of-mass collision energies.
ligand switching are all exothermic or thermoneutral for most of the clusters under the conditions of our experiment. We present the collisionally activated regeneration of Nb3 Oÿ 8 from in Fig. 5. the tertiary product ion Nb3 O9 C4 Hÿ 10 This ®gure clearly illustrates there are no intermediate decomposition products, so it is apparent that ligand coupling proceeds on the cluster before activation. If it takes place during activation, statistical arguments could be used to rationalize a very small activation energy for this reaction.
We are currently exploring other anionic metal oxide systems for analogous behavior. Acknowledgements We gratefully acknowledge ®nancial support from the Australian Research Council, the Australian Institute of Nuclear Science and Engineering, and the Nuclear Medicine Research Foundation, Concord Hospital, Sydney, Australia.
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